Distance sampling for Sonoran Desert tortoises. Swann, D., Averill-Murray, R., & Schwalbe, C. Journal of Wildlife Management, 66(4):969–975, U.S. Geological Survey, Sonoran Desert Field Station, University of Arizona, Tucson, AZ 85721, United States, 2002. abstract bibtex We used line transects and distance sampling in combination with radiotelemetry to estimate density of a desert tortoise (Gopherus agassizii) population in the Rincon Mountains near Tucson, Arizona, USA, as part of a long-term study evaluating the impact of urban development on tortoises. During 2000, 34 1-km transects were each sampled twice in the 368.5-ha study area. We observed 46 tortoises with midline carapace lengths ?150 mm (subadults and adults) plus 7 juveniles on transects. For subadults and adults, the encounter rate was 0.63 tortoises/km, and the mean proportion of tortoises observable during radiotelemetry, conducted concurrently with transect sampling, was 82%. Corrected mean density based on line transects and radiotelemetry was 0.523 tortoises/ha (CV = 22.99, 95% CI = 0.29-0.79), and absolute abundance in the study area was estimated to be 193 (CV = 23.0%, CI = 107-291). Using the 2 independent coverages of transects as separate samples, the Lincoln-Petersen mark-recapture estimator produced an abundance estimate of 224 subadult and adult tortoises (CV = 53.9%, CI = 72-440). Transects measured on the ground over uneven topography resulted in 3% smaller estimates of density when compared to analysis with transect lengths determined from coordinates plotted on a map. Distance sampling appears to be a feasible method of estimating density of Sonoran Desert populations of the desert tortoise, but transect lengths should be based on mapped rather than measured distances to prevent biases caused by uneven topography.
@ARTICLE{Swann2002,
author = {Swann, D.E. and Averill-Murray, R.C. and Schwalbe, C.R.},
title = {Distance sampling for Sonoran Desert tortoises},
journal = {Journal of Wildlife Management},
year = {2002},
volume = {66},
pages = {969--975},
number = {4},
abstract = {We used line transects and distance sampling in combination with radiotelemetry
to estimate density of a desert tortoise \textit{(Gopherus agassizii)}
population in the Rincon Mountains near Tucson, Arizona, USA, as
part of a long-term study evaluating the impact of urban development
on tortoises. During 2000, 34 1-km transects were each sampled twice
in the 368.5-ha study area. We observed 46 tortoises with midline
carapace lengths ?150 mm (subadults and adults) plus 7 juveniles
on transects. For subadults and adults, the encounter rate was 0.63
tortoises/km, and the mean proportion of tortoises observable during
radiotelemetry, conducted concurrently with transect sampling, was
82%. Corrected mean density based on line transects and radiotelemetry
was 0.523 tortoises/ha (CV = 22.99, 95% CI = 0.29-0.79), and absolute
abundance in the study area was estimated to be 193 (CV = 23.0%,
CI = 107-291). Using the 2 independent coverages of transects as
separate samples, the Lincoln-Petersen mark-recapture estimator produced
an abundance estimate of 224 subadult and adult tortoises (CV = 53.9%,
CI = 72-440). Transects measured on the ground over uneven topography
resulted in 3% smaller estimates of density when compared to analysis
with transect lengths determined from coordinates plotted on a map.
Distance sampling appears to be a feasible method of estimating density
of Sonoran Desert populations of the desert tortoise, but transect
lengths should be based on mapped rather than measured distances
to prevent biases caused by uneven topography.},
address = {U.S. Geological Survey, Sonoran Desert Field Station, University
of Arizona, Tucson, AZ 85721, United States},
file = {Swannetal2002.pdf:Swannetal2002.pdf:PDF},
keywords = {Abundance estimation, Arizona, Density estimation, Desert tortoise,
Distance sampling, Gopherus agassizii, Line transect, Radiotelemetry,
Sonoran Desert},
owner = {eric},
subdatabase = {distance},
timestamp = {2006.11.05}
}
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During 2000, 34 1-km transects were each sampled twice in the 368.5-ha study area. We observed 46 tortoises with midline carapace lengths ?150 mm (subadults and adults) plus 7 juveniles on transects. For subadults and adults, the encounter rate was 0.63 tortoises/km, and the mean proportion of tortoises observable during radiotelemetry, conducted concurrently with transect sampling, was 82%. Corrected mean density based on line transects and radiotelemetry was 0.523 tortoises/ha (CV = 22.99, 95% CI = 0.29-0.79), and absolute abundance in the study area was estimated to be 193 (CV = 23.0%, CI = 107-291). Using the 2 independent coverages of transects as separate samples, the Lincoln-Petersen mark-recapture estimator produced an abundance estimate of 224 subadult and adult tortoises (CV = 53.9%, CI = 72-440). Transects measured on the ground over uneven topography resulted in 3% smaller estimates of density when compared to analysis with transect lengths determined from coordinates plotted on a map. Distance sampling appears to be a feasible method of estimating density of Sonoran Desert populations of the desert tortoise, but transect lengths should be based on mapped rather than measured distances to prevent biases caused by uneven topography.","address":"U.S. Geological Survey, Sonoran Desert Field Station, University of Arizona, Tucson, AZ 85721, United States","file":"Swannetal2002.pdf:Swannetal2002.pdf:PDF","keywords":"Abundance estimation, Arizona, Density estimation, Desert tortoise, Distance sampling, Gopherus agassizii, Line transect, Radiotelemetry, Sonoran Desert","owner":"eric","subdatabase":"distance","timestamp":"2006.11.05","bibtex":"@ARTICLE{Swann2002,\r\n author = {Swann, D.E. and Averill-Murray, R.C. and Schwalbe, C.R.},\r\n title = {Distance sampling for Sonoran Desert tortoises},\r\n journal = {Journal of Wildlife Management},\r\n year = {2002},\r\n volume = {66},\r\n pages = {969--975},\r\n number = {4},\r\n abstract = {We used line transects and distance sampling in combination with radiotelemetry\r\n\tto estimate density of a desert tortoise \\textit{(Gopherus agassizii)}\r\n\tpopulation in the Rincon Mountains near Tucson, Arizona, USA, as\r\n\tpart of a long-term study evaluating the impact of urban development\r\n\ton tortoises. During 2000, 34 1-km transects were each sampled twice\r\n\tin the 368.5-ha study area. We observed 46 tortoises with midline\r\n\tcarapace lengths ?150 mm (subadults and adults) plus 7 juveniles\r\n\ton transects. For subadults and adults, the encounter rate was 0.63\r\n\ttortoises/km, and the mean proportion of tortoises observable during\r\n\tradiotelemetry, conducted concurrently with transect sampling, was\r\n\t82%. Corrected mean density based on line transects and radiotelemetry\r\n\twas 0.523 tortoises/ha (CV = 22.99, 95% CI = 0.29-0.79), and absolute\r\n\tabundance in the study area was estimated to be 193 (CV = 23.0%,\r\n\tCI = 107-291). Using the 2 independent coverages of transects as\r\n\tseparate samples, the Lincoln-Petersen mark-recapture estimator produced\r\n\tan abundance estimate of 224 subadult and adult tortoises (CV = 53.9%,\r\n\tCI = 72-440). 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